1. Field of the Invention
This invention relates to sheet separating mechanisms. It is especially useful with sheet separating mechanisms that separate the bottom sheet from a stack of sheets that have freshly fixed toner images on their topsides.
2. Description of the Prior Art
Many commercial copiers use an intermediate tray, sometimes called a duplex tray, for receiving copy sheets that have had one image transferred and fixed to them. These sheets are later fed out of the intermediate tray to receive a second image as properly controlled by a logic and control unit in the copier. This intermediate tray can be used to make duplex copies and to make color copies. In the duplex case, the opposite sides of the sheet receive the images, whereas with two-color copies the same side of the sheet receives two images. Duplex two-color copies can be made by feeding the sheet through the intermediate tray three times.
Sheets fed out of the intermediate tray are generally separated from the bottom of the stack. Separating from the bottom is a difficult task. As a result, vacuum rollers have been used for that purpose because of their reliability. However, vacuum rollers are expensive in both power and equipment and may increase the noise output of the copier. Scuff feeding devices are much quieter and less expensive, but have a tendency to be less reliable.
Typical scuff sheet separating devices include a belt or roller which engage the bottom sheet in a stack and feed it along a path. A scuff separating pad or roller is mounted on the opposite side of the path and prevents a second sheet from being fed. In the classic scuff separating device, the coefficient of friction of the belt or roller which is feeding the bottom sheet is higher than that of the scuff separating pad or roller which in turn is higher than the friction between the sheets. Thus, the bottom sheet slides on the next sheet while the next sheet is held by the scuff pad. When only one sheet is left in the stack, that sheet slides on the scuff pad.
U.S. Pat. No. 4,844,435 to Giannetti et al., issued July 4, 1989, describes a scuff separating device which feeds document sheets from the bottom of a stack using a retard roller which is braked. The brake on the retard roller prevents rotation of the roller when more than one sheet is in the nip, but is overcome and permits rotation when one or no sheets are in the nip. This structure is extremely reliable in preventing double feeds and permitting the passage of a single sheet in highly demanding document feeding applications.
Each of the above scuff separating devices rely for separation on a certain amount of force of the scuff separating pad or roller on the top of the second sheet. Unfortunately, in a duplex mode, the top of the first sheet has a freshly fixed toner image, which includes particles of toner that can be disturbed in response to rubbing. With scuff feeding devices of each of the types described, the leading millimeter or so of the bottom of the second sheet is vigorously rubbed by the entire top surface of the bottom sheet. Unless the image on the top surface of the bottom sheet is extremely well fixed, toner will rub off on the front portion of the bottom of the second sheet creating a toner mark across that edge of the sheet and smudging the image on the bottom sheet.